In a printer, a copier or other imaging system, paper or other media is loaded as a stack of cut sheets. For example, blank paper or other recording media is loaded into one or more input trays so that it can be printed. How much media is left in the input tray is not always readily apparent to the user because of the design and location of the input tray. Yet the information of how much media remains is useful for managing the printing operation, as well as for an early warning that more media will be needed to be supplied. As a first example, suppose a user requests a print job requiring 20 sheets of media, but only 10 sheets are in the input tray. If the user leaves the printing job unattended and comes back later, he will be disappointed to find that the printing job is unfinished because the printer ran out of paper. In addition, while waiting for the job to continue, the printhead may return to the maintenance station and expel additional ink that would not have occurred otherwise. As a second example, if a user has a job that needs to be printed, but does not realize he is almost out of paper, he may need to make a special trip to get more, thus causing delays in printing the job. In this example, an early warning would be helpful so that the user can get more paper before his local supply runs out.
Media stack height detectors have been disclosed in the prior art, for example U.S. Pat. Nos. 5,839,015 and 7,374,163. However, competitive pressures make it desirable to incorporate the function of media stack height detection at low cost. Prior art media stack height detectors typically use an extra coupling component that contacts the top sheet of the media stack at one end and has a portion that provides a stack-height-dependent signal to a corresponding sensor at another other end. The extra coupling component not only adds cost to the system, but also its contacting the sheet of media adds drag as the sheet is being moved from the media input tray into a position for printing. Thus an improved apparatus and method for detecting media stack height is needed.